1. Field of the Invention
The present invention relates to a liquid crystal display and, more particularly, to an electrostatic discharge protection device for a liquid crystal display using a chip on glass (COG) package, which protects the input pads of the display from electrostatic charge.
2. Discussion of Related Art
In a thin film transistor-liquid crystal display (TFT-LCD), packaging techniques for connecting a driving IC, which provides a driving signal to the LCD to the TFT-LCD panel, include wire bonding (WB), tape automated bonding (TAB) and chip on glass (COG) methods. In the WB method, the driving ICs are connected to the panel electrodes with Au wires. The TAB method mounts a package, in which the driving IC is connected to a film carrier, on the panel. Finally, the COG method forms a bump on a bare chip and mounts it on the panel in which inner and outer lead pads are formed.
During a TFT-LCD fabricating process, static electricity may be applied to the TFT-LCD, destroying its inner elements, such as a TFT. To prevent the damaging effect of the static electricity after forming the TFT-LCD array, a shorting bar is used to facilitate the inspection of the TFT-LCD array. The shorting bar structure consists of one shorting bar connected to each gate line of the TFT-LCD and another shorting bar connected to each data line. To check if the gate line is short-circuited during the array inspection, the power is applied to the shorting bar connected to each gate line and the opposite end of the gate line is checked. The data line is checked using the shorting bar in the same manner as the gate line.
The shorting bar is not used in actual driving of the TFT-LCD but used only for electrostatic discharge protection and array inspection. The shorting bar is then disconnected from the TFT-LCD array. That is, after the TFT-LCD array is fabricated and its inspection is completed, the upper and lower plates of the TFT-LCD are attached to each other and a scribe process and a grinding process are sequentially carried out. Thereafter, the shorting bar is disconnected. Meanwhile, it is difficult to form the shorting bar according to the packaging methods since the configurations of the gate lines and data lines depend on the packaging methods. In other words, with the TAB method, the carrier film is connected to both sides of the bare chip so that a space on the TFT-LCD panel can be secured. It allows the shorting bars to be formed on one side of the gate line pad and one side of the data line pad which are also the areas reserved for placing the driving ICs.
On the other hand, with the COG method, a space on the TFT-LCD panel is difficult to secure because the inner and outer leads must be formed on the driving IC mounting areas of the panel. Thus, the shorting bar cannot be formed on the driving IC mounting areas. To solve this problem, there has been developed a technique for forming the shorting bar on the TFT-LCD in COG package. This technique is explained below with reference to the attached drawings.
FIG. 1 is a layout of a conventional liquid crystal display in a COG package having a shorting bar. FIG. 2 is a layout of a conventional liquid crystal display in a COG package having an electrostatic discharge protecting circuit. Referring to FIG. 1, a plurality of gate lines 1 are arranged in one direction on a substrate at regular intervals, and a plurality of data lines 2 are arranged at regular intervals perpendicular to the gate lines 1. A gate line pad 3 and a data line pad 4 are formed at one end of each gate lines 1 and one end of each data line 2, respectively. A plurality of gate line input pads 7 are formed at regular intervals opposite to gate line pads 3, and a plurality of data line input pads 8 are arranged at regular intervals opposite to data line pads 4. The gate driving IC mounting area corresponds to the region of the gate line input pads 7, and data driving IC mounting area corresponds to the region of the data input pads 8.
First and second shorting bars 5 and 6 are formed on the gate driving IC mounting area 9 and the data driving IC mounting area 10, respectively. That is, a first shorting bar 5 is formed between the gate line pads 3 and gate input pads 7 and connected to each of the gate line pads 3. A second shorting bar 6 is formed between the data line pads 4 and the data input pads 8 and connected to each of the data line pads 4. Reference numeral 11 denotes a laser cutting line. After the TFT-LCD is constructed as above, which includes TFTs and pixel electrodes, an inspection of the array is performed, followed by the attachment of the upper and lower plates with respect to each other. Then, portions between shorting bars 5 and 6 and gate line pads 3 and data line pads 4 are cut using a laser cutting apparatus.
Referring to FIG. 2, the conventional TFT-LCD using a COG package, having an electrostatic discharge protection circuit, is constructed in such a manner that a plurality of gate lines 1 are arranged in one direction on a substrate at regular intervals, a plurality of data lines 2 are arranged at regular intervals perpendicular to the gate lines 1, and a common electrode 12 is formed around the TFT-LCD. A gate line pad 3 and data line pad 4 are formed at one end of each gate lines 1 and one end of each data line 2, respectively. A plurality of gate line input pads 7 are formed at regular intervals opposite to gate line pads 3, and a plurality of data input pads 8 are arranged at regular intervals opposite to the data line pads 4. Both ends of the gate line 1 and the data line 2 are connected to the common electrode 12 through an electrostatic discharge protection circuit 13. The gate driving IC mounting area corresponds to the region of the gate line input pads 7, and the data driving IC mounting area corresponds to the region of the data input pads 8. The common electrode 12 is connected to a common electrode (not shown) of the upper plate of the display using an Ag dot.
The conventional TFT-LCD in a COG package has the following problems. While the inner circuit of the TFT-LCD is protected from electrostatic discharge because either the shorting bar or the electrostatic discharge protection circuit is provided to the outer lead pad placed on the TFT-LCD array side, the input pads are all floating so that there is no path through which static electricity is discharged. Thus, the input pads are susceptible to and can be damaged by the electrostatic discharge. Furthermore, after ICs are attached to the input/output pads, static electricity generated from the ICs remains in the input pad, damaging the ICs with the electrostatic discharge.